The internal dimensions within integrated circuits continue to shrink, including the thicknesses of the dielectric layers used to separate various layers of wiring from one another. However, as these wiring levels are brought closer together, the possibility of cross-coupling between them starts to rise as well as the incidence of parasitic capacitances. One way to minimize this problem is to reduce the dielectric constants of these inter-metal layers. Thus there is considerable interest in developing low k materials as well as deposition methods for them that are compatible with integrated circuit technology.
For our purposes we will define a low k dielectric as one that has a dielectric constant dose to or less than about 3. Several such materials are known to exist but they have the disadvantage that they are organic rather than inorganic compounds. Examples include hydrogen silsesquioxane, fluorinated polyimide, polyarylene ether, fluorinated arylene ether, polytetrafluoro-ethylene, and benzocyclobutene. Because of their organic nature these materials are innately soft, which physical property can give rise to problems during semiconductor processing, particularly during planarization.
The present invention is concerned with a low dielectric constant material that is essentially inorganic in nature, referred to by practitioners of the art as black diamond, or BD. BD is silica that has been doped with about 10 mole % methane. When formed as will be described below, BD is porous, about 36% of its volume being in the form of pores having a diameter between about 8 and 24 Angstroms.
To be of acceptable quality, the resistivity of an inter-metal dielectric layer needs to exceed about 10.sup.5 ohm-cm. In practice an indication of the `leakiness` of any given film can be obtained by measuring its flat-band voltage. This is the gate voltage required tp bring about the flat-band condition, so the higher the flat-band voltage, the greater the leakage current associated with the film.,
Two processes have been used in the prior art to form BD films of acceptable quality. In the hot film method, a mixture of trimethyl silane, silane, nitrous oxide, and oxygen is used at about 400.degree. C. to form a plasma out of which the BD deposits. In the cold film method, deposition of the BD film takes place from a room temperature plasma of a mixture of helium, methyl silane, and nitrous oxide.
It has been found that films deposited by the cold film method are, as deposited, of poor quality. To raise the quality to acceptable levels, the practice of the prior art has been to first stabilize them by heating in oxygen for about 40 minutes at about 200.degree. C. followed by further heating in nitrogen for about 30 minutes at about 400.degree. C. This post-deposition treatment adds to the overall manufacturing cost, both because of the extra energy that must be expended (increased thermal budget) as well as because of the associated reduction of product throughput in the manufacturing line.
A routine search of the prior art was performed but no references that teach the exact processes and structures of the present invention were discovered. Several references of interest were, however, encountered along the way. For example, in U.S. Pat. No. 5,504,040, Moslehi shows a dielectric oxide process that uses a chuck chilling device while in U.S. Pat. No. 4,992,306 Hochberg teaches a deposition process for silicon oxide low k using organic reactants. Reference to BD was found on the web-site for Applied Materials in the form of a press release dated Feb. 28, 2000 describing this material.